High bandwidth apparatus and method for generating differential signals

ABSTRACT

An apparatus and method for generating differential signals. The apparatus includes a first operational amplifier receiving a first signal, a second operational amplifier receiving a second signal, and a first transistor. The first transistor includes a first gate, a first terminal, and a second terminal. Additionally, the apparatus includes a second transistor. The second transistor includes a second gate, a third terminal, and a fourth terminal. Moreover, the apparatus includes a first resistor coupled to the first terminal and the third terminal, and a second resistor coupled to the second terminal and the fourth terminal. Also, the apparatus includes a first current supplier coupled to the first terminal, a second current supplier coupled to the second terminal, a third current supplier coupled to the third terminal, and a fourth current supplier coupled to the fourth terminal.

CROSS-REFERENCES TO RELATED APPLICATIONS

This application is a continuation of U.S. patent Ser. No. 11/678,467,filed Feb. 23, 2007, entitled “High Bandwidth Apparatus and Method forGenerating Differential Signals,” by Inventors Wenzhe Luo and PaulOuyang, which is a continuation of U.S. patent Ser. No. 11/156,236, nowU.S. Pat. No. 7,205,839, filed Jun. 15, 2005, entitled “High BandwidthApparatus and Method for Generating Differential Signals,” by InventorsWenzhe Luo and Paul Ouyang, which claims priority to Chinese PatentApplication No. 200510026471.0, filed May 30, 2005, entitled “HighBandwidth Apparatus and Method for Generating Differential Signals,” byInventors Wenzhe Luo and Paul Ouyang, commonly assigned, incorporated byreference herein for all purposes.

STATEMENT AS TO RIGHTS TO INVENTIONS MADE UNDER FEDERALLY SPONSOREDRESEARCH OR DEVELOPMENT

Not Applicable

REFERENCE TO A “SEQUENCE LISTING,” A TABLE, OR A COMPUTER PROGRAMLISTING APPENDIX SUBMITTED ON A COMPACT DISK

Not Applicable

BACKGROUND OF THE INVENTION

The present invention is directed to integrated circuits. Moreparticularly, the invention provides an apparatus and method forgenerating differential signals. Merely by way of example, the inventionhas been applied to an analog-to-digital converter. But it would berecognized that the invention has a much broader range of applicability.

The analog-to-digital converters have been widely used for converting ananalog input signal to a digital output signal. For example, theanalog-to-digital converters are often required for providing analogsignals to digital circuits. The analog input signals are usuallycontinuous in time and amplitude, and the digital output signals areoften discrete in time and amplitude.

Some analog-to-digital converters each include a comparator tosuccessively compare an analog signal to several reference signals.These converters are often slow because a series of comparisons usuallyhas to be performed in order to convert the analog signal to a digitalsignal. Other analog-to-digital converters are flash converters. Theflash converters often use many comparators to simultaneously compare ananalog signal to several reference signals. These comparators cansimultaneously provide bits for a digital signal.

The conventional analog-to-digital converters often use differentialmechanism. The differential mechanism converts a single-ended inputsignal to differential input signals. For example, the differentialconversion is performed by a charge and transfer circuit. The charge andtransfer circuit usually performs well for low and medium frequencies,but for high sampling frequencies, such as one that is higher than 100MHz, the charge and transfer circuit often cannot easily achieveconversion accuracy better than eight bits. In another example, thedifferential conversion is performed by a high-speed operationalamplifier. The operational amplifier often consumes significant currentand introduces extra noises. In yet another example, the differentialconversion is accomplished by a transformer. But the transformer cannotbe easily integrated onto a digital circuit chip in many applications.

From the above, it is seen that an improved technique for generatingdifferential signals is desired.

BRIEF SUMMARY OF THE INVENTION

The present invention is directed to integrated circuits. Moreparticularly, the invention provides an apparatus and method forgenerating differential signals. Merely by way of example, the inventionhas been applied to an analog-to-digital converter. But it would berecognized that the invention has a much broader range of applicability.

In a specific embodiment, the invention provides an apparatus forgenerating differential signals is provided. The apparatus includes afirst operational amplifier receiving a first signal, a secondoperational amplifier receiving a second signal, and a first transistor.The first transistor includes a first gate, a first terminal, and asecond terminal. Additionally, the apparatus includes a secondtransistor. The second transistor includes a second gate, a thirdterminal, and a fourth terminal. Moreover, the apparatus includes afirst resistor coupled to the first terminal and the third terminal, anda second resistor coupled to the second terminal and the fourthterminal. Also, the apparatus includes a first current supplier coupledto the first terminal, a second current supplier coupled to the secondterminal, a third current supplier coupled to the third terminal, and afourth current supplier coupled to the fourth terminal. The gate of thefirst transistor is coupled to the first operational amplifier, the gateof the second transistor is coupled to the second operational amplifier,the second terminal is configured to output a third signal, and thefourth terminal is configured to output a fourth signal. A firstdifference between the fourth signal and the third signal issubstantially proportional to a second difference between the firstsignal and the second signal.

In another embodiment, an apparatus for generating differential signalsincludes a first operational amplifier receiving a first signal, asecond operational amplifier receiving a second signal, and a firsttransistor. The first transistor includes a first gate, a firstterminal, and a second terminal. Additionally, the apparatus includes asecond transistor. The second transistor includes a second gate, a thirdterminal, and a fourth terminal. Moreover the apparatus includes a firstresistor coupled to the first terminal and the third terminal, and asecond resistor coupled to the second terminal and the fourth terminal.Also, the apparatus includes a first current supplier coupled to thefirst terminal, a second current supplier coupled to the secondterminal, a third current supplier coupled to the third terminal, and afourth current supplier coupled to the fourth terminal. Additionally,the apparatus includes a common-mode feedback device. The gate of thefirst transistor is coupled to the first operational amplifier, and thegate of the second transistor is coupled to the second operationalamplifier. The second terminal is configured to output a third signal,and the fourth terminal is configured to output a fourth signal. Thecommon-mode feedback device is configured to receive at least the thirdsignal and the fourth signal, and the common-mode feedback device isconfigured to adjust the second current supplier and the fourth currentsupplier. A first difference between the fourth signal and the thirdsignal is substantially proportional to a second difference between thefirst signal and the second signal.

According to yet another embodiment, a method for generatingdifferential signals includes receiving a first signal and a secondsignal, and generating a third signal and a fourth signal. The thirdsignal is substantially equal to the first signal, and the fourth signalis substantially equal to the second signal. Additionally, the methodincludes converting a first voltage difference between the third signaland the fourth signal to a first current, and generating a secondcurrent. The second current is substantially the same as the firstcurrent in magnitude. Moreover, the method includes converting thesecond current to a second voltage difference between a fifth signal anda sixth signal. The first voltage difference is proportional to thesecond voltage difference.

Many benefits are achieved by way of the present invention overconventional techniques. Some embodiments of the present inventionconvert voltage signals to differential current signals and then convertthe differential current signals to differential voltage signals. Forexample, symmetrical high-bandwidth unity-gain buffers are used. Certainembodiments of the present invention consume little power. Someembodiments of the present invention provide high bandwidth for inputvoltage signals. Certain embodiments of the present invention do not usea clock signal and therefore do not rely on sampling frequency. Someembodiments of the present invention can be easily integrated into adigital circuit chip. Certain embodiments of the present invention canprovide differential voltage signals for many applications, such asdifferential flash analog-to-digital converter. Depending upon theembodiment, one or more of these benefits may be achieved. These andother benefits will be described in more throughout the presentspecification and more particularly below.

Various additional objects, features and advantages of the presentinvention can be more fully appreciated with reference to the detaileddescription and accompanying drawings that follow.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a simplified apparatus for generating differential signalsaccording to an embodiment of the present invention;

FIG. 2 is a simplified method for generating differential signalsaccording to an embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention is directed to integrated circuits. Moreparticularly, the invention provides an apparatus and method forgenerating differential signals. Merely by way of example, the inventionhas been applied to an analog-to-digital converter. But it would berecognized that the invention has a much broader range of applicability.

FIG. 1 is a simplified apparatus for generating differential signalsaccording to an embodiment of the present invention. This diagram ismerely an example, which should not unduly limit the scope of the claimsherein. The apparatus 100 includes the following components:

1. Operational Amplifiers 110 and 112;

2. Current suppliers 120, 122, 124, and 126;

3. Resistors 130 and 132;

4. Transistors 140 and 142;

5. Common-Mode Feedback Device 150.

The above electronic devices provide components for the apparatus 100according to an embodiment of the present invention. Other alternativescan also be provided where certain devices are added, one or moredevices are removed, or one or more devices are arranged with differentconnections without departing from the scope of the claims herein.

The operational amplifiers 110 and 112 receive input signals 210 and 212at their positive terminals respectively. For example, the input signals210 and 212 are analog signals. In another example, the operationalamplifiers 110 and 112 are the same. As shown in FIG. 1, the outputsignal 220 of the operational amplifier 110 is received by a sourcefollower 230, and the output signal 222 of the operational amplifier 112is receive by another source follower 232. The source follower 230includes the current supplier 120 and the transistor 140, and the sourcefollower 232 includes the current supplier 122 and the transistor 142.For example, the transistor 140 is an NMOS transistor, and thetransistor 142 is another NMOS transistor. In another example, thetransistors 140 and 142 are the same. In yet another example, thecurrent suppliers 120 and 122 are the same. In yet another example, thecurrent suppliers 120 and 122 are coupled to a voltage source of V_(SS).In yet another example, the source followers 230 and 232 are the same.

According to one embodiment, the operational amplifier 110 and thesource follower 230 are configured as a unity gain follower, and theoperational amplifier 112 and the source follower 232 are configured asanother unity gain follower. The voltage level at a node 240 follows thesignal 210, and the voltage level at a node 242 follows the signal 212.For example, the operational amplifiers 110 and 112 each have smallcapacitor loading. In another example, the voltage level at the node 240duplicates the voltage level of the signal 210 within a high bandwidth,and the voltage level at the node 242 also duplicates the voltage levelof the signal 212 within a high bandwidth. As shown in FIG. 1, thevoltage at the node 240 is received by the operational amplifier 110,and the voltage at the node 242 is received by the operational amplifier112.

The resistor 130 is coupled between the nodes 240 and 242. The currentI_(R1) flowing through the resistor 130 is determined as follows:

$\begin{matrix}{I_{R\; 1} = \frac{V_{A} - V_{B}}{R_{1}}} & \left( {{Equation}\mspace{14mu} 1} \right)\end{matrix}$

where V_(A) and V_(B) represent the voltage levels at the nodes 240 and242 respectively. If V_(IP) and V_(IN) represent the voltage levels ofthe signals 210 and 212 respectively, V_(IP) is equal to V_(A) andV_(IN) is equal to V_(B) according to an embodiment of the presentinvention. The current I_(R1) loads the source followers 230 and 232,and results in another current I_(R2) between nodes 250 and 252.

The nodes 250 and 252 are connected by the resistor 132 and are coupledto the current suppliers 126 and 124 respectively. The current I_(R2)flows from the node 252 to the node 250 through the resistor 132. Forexample, the current suppliers 126 and 124 are the same. In anotherexample, the current suppliers 126 and 124 are coupled to a voltagesource of V_(DD). As shown in FIG. 1, the current I_(R2) is equal to thecurrent I_(R1), so the voltage difference between the nodes 252 and 250is as follows:

$\begin{matrix}{{V_{OP} - V_{ON}} = {\frac{R_{2}}{R_{1}} \times \left( {V_{A} - V_{B}} \right)}} & \left( {{Equation}\mspace{14mu} 2} \right)\end{matrix}$

where V_(OP) and V_(ON) represent the voltages levels at the nodes 252and 250 respectively. As discussed above, V_(IP) is equal to V_(A) andV_(IN) is equal to V_(B) according to an embodiment of the presentinvention. Therefore, the difference between V_(OP) and V_(ON) iscalculated as follows:

$\begin{matrix}{{V_{OP} - V_{ON}} = {\frac{R_{2}}{R_{1}} \times \left( {V_{IP} - V_{IN}} \right)}} & \left( {{Equation}\mspace{14mu} 3} \right)\end{matrix}$

For example, R₁ is equal to R₂. Accordingly,V_(OP)−V_(ON)=V_(IP)−V_(IN).

As shown in FIG. 1, V_(OP) and V_(ON) represent voltage levels ofsignals 260 and 262 respectively. The signals 260 and 262 aredifferential output signals, which are regulated by the common-modefeedback device 150. The common-mode feedback device 150 receives thesignals 260 and 262, and a signal 152. The signal 152 is biased to avoltage level of V_(m0). In one embodiment, a common mode voltage is anaverage of V_(OP) and V_(ON). In another embodiment, the common-modefeedback device 150 is used to adjust the common mode voltage. Forexample, the voltage level of V_(m0) is predetermined. In anotherexample, the voltage level of V_(m0) is used as a common-mode voltage.Additionally, the common-mode feedback device 150 controls the currentsuppliers 126 and 124.

In one embodiment, the common-mode feedback device 150 receives thesignals 260 and 262, and determines the average voltage level of V_(OP)and V_(ON). This average voltage level is compared with the voltagelevel of V_(m0). If the average voltage level is different from thevoltage level of V_(m0), the common-mode feedback device adjusts thecurrent suppliers 124 and 126 and thereby changes the signals 260 and262 until the average voltage level becomes the same as the voltagelevel of V_(m0). Accordingly, V_(OP) and V_(ON) are determined asfollows:

$\begin{matrix}{V_{OP} = {V_{m\; 0} + {\frac{1}{2}\left( {V_{ip} - V_{in}} \right)}}} & \left( {{Equation}\mspace{14mu} 4} \right) \\{V_{ON} = {V_{m\; 0} - {\frac{1}{2}\left( {V_{ip} - V_{in}} \right)}}} & \left( {{Equation}\mspace{14mu} 5} \right)\end{matrix}$

As shown in Equations 4 and 5, the sum of V_(OP) and V_(ON) is equal totwo times of V_(m0).

As discussed above and further emphasized here, FIG. 1 is merely anexample, which should not unduly limit the scope of the claims. One ofordinary skill in the art would recognize many variations, alternatives,and modifications. For example, the apparatus 100 can be used for asingle-ended input signal or for differential input signals. Fordifferential input signals, the input signals are connected as thesignals 210 and 212. For single-ended input signal, the input signal isconnected as one of the signals 210 and 212. The other one of thesignals 210 and 212 is connected to a predetermined voltage level. Forexample, the predetermined voltage level is an input common-mode level.In another example, the predetermined voltage level is equal to 0 volt.

FIG. 2 is a simplified method for generating differential signalsaccording to an embodiment of the present invention. This diagram ismerely an example, which should not unduly limit the scope of the claimsherein. The method 300 includes the following processes:

1. Process 310 for duplicating input signals;

2. Process 320 for converting voltage difference into current;

3. Process 330 for converting current to differential voltage signals;

4. Process 340 for determining common-mode voltage.

The above sequence of processes provides a method according to anembodiment of the present invention. Other alternatives can also beprovided where processes are added, one or more processes are removed,or one or more processes are provided in a different sequence withoutdeparting from the scope of the claims herein. For example, theprocesses 330 and 340 are performed at the same time. Future details ofthe present invention can be found throughout the present specificationand more particularly below.

For example, the method 300 can be performed by the apparatus 100according to an embodiment of the present invention. At the process 310,the signal 210 is duplicated to the node 240 through the operationalamplifier 110 and the source follower 230, and the signal 212 isduplicated to the node 242 through the operational amplifier 112 and thesource follower 232.

At the process 320, the voltage difference between the nodes 240 and 242is converted to the current I_(R1) that flows through the resistor 130.At the process 330, the current I_(R1) is duplicated into the currentI_(R2). The current I_(R2) flows through the resistor 132, and convertsthe voltage difference between the signals 210 and 212 into the voltagedifference between the signals 260 and 262. At the process 340, thecommon-mode voltage for the signals 260 and 262 are determined by thecommon-mode feedback device 150 through the current suppliers 124 and126.

As discussed above and further emphasized here, FIG. 2 is merely anexample, which should not unduly limit the scope of the claims. One ofordinary skill in the art would recognize many variations, alternatives,and modifications. For example, the method 300 can be used for asingle-ended input signal or for differential input signals. Fordifferential input signals, the input signals are connected as thesignals 210 and 212. For single-ended input signal, the input signal isconnected as one of the signals 210 and 212. The other one of thesignals 210 and 212 is connected to an input common-mode level. Forexample, the input common-mode level is equal to 0 volt.

In another embodiment, an apparatus for generating differential signalsincludes a first operational amplifier receiving a first signal, asecond operational amplifier receiving a second signal, and a firsttransistor. The first transistor includes a first gate, a firstterminal, and a second terminal. Additionally, the apparatus includes asecond transistor. The second transistor includes a second gate, a thirdterminal, and a fourth terminal. Moreover, the apparatus includes afirst resistor coupled to the first terminal and the third terminal, anda second resistor coupled to the second terminal and the fourthterminal. Also, the apparatus includes a first current supplier coupledto the first terminal, a second current supplier coupled to the secondterminal, a third current supplier coupled to the third terminal, and afourth current supplier coupled to the fourth terminal. The gate of thefirst transistor is coupled to the first operational amplifier, the gateof the second transistor is coupled to the second operational amplifier,the second terminal is configured to output a third signal, and thefourth terminal is configured to output a fourth signal. A firstdifference between the fourth signal and the third signal issubstantially proportional to a second difference between the firstsignal and the second signal. For example, the apparatus is implementedaccording to the apparatus 100.

In another embodiment, an apparatus for generating differential signalsincludes a first operational amplifier receiving a first signal, asecond operational amplifier receiving a second signal, and a firsttransistor. The first transistor includes a first gate, a firstterminal, and a second terminal. Additionally, the apparatus includes asecond transistor. The second transistor includes a second gate, a thirdterminal, and a fourth terminal. Moreover the apparatus includes a firstresistor coupled to the first terminal and the third terminal, and asecond resistor coupled to the second terminal and the fourth terminal.Also, the apparatus includes a first current supplier coupled to thefirst terminal, a second current supplier coupled to the secondterminal, a third current supplier coupled to the third terminal, and afourth current supplier coupled to the fourth terminal. Additionally,the apparatus includes a common-mode feedback device. The gate of thefirst transistor is coupled to the first operational amplifier, and thegate of the second transistor is coupled to the second operationalamplifier. The second terminal is configured to output a third signal,and the fourth terminal is configured to output a fourth signal. Thecommon-mode feedback device is configured to receive at least the thirdsignal and the fourth signal, and the common-mode feedback device isconfigured to adjust the second current supplier and the fourth currentsupplier. A first difference between the fourth signal and the thirdsignal is substantially proportional to a second difference between thefirst signal and the second signal. For example, the apparatus isimplemented according to the apparatus 100.

According to yet another embodiment, a method for generatingdifferential signals includes receiving a first signal and a secondsignal, and generating a third signal and a fourth signal. The thirdsignal is substantially equal to the first signal, and the fourth signalis substantially equal to the second signal. Additionally, the methodincludes converting a first voltage difference between the third signaland the fourth signal to a first current, and generating a secondcurrent. The second current is substantially the same as the firstcurrent in magnitude. Moreover, the method includes converting thesecond current to a second voltage difference between a fifth signal anda sixth signal. The first voltage difference is proportional to thesecond voltage difference. For example, the method is implementedaccording to the method 300.

The present invention has various advantages. Some embodiments of thepresent invention convert voltage signals to differential currentsignals and then convert the differential current signals todifferential voltage signals. For example, symmetrical high-bandwidthunity-gain buffers are used. Certain embodiments of the presentinvention consume little power. Some embodiments of the presentinvention provide high bandwidth for input voltage signals. Certainembodiments of the present invention do not use a clock signal andtherefore do not rely on sampling frequency. Some embodiments of thepresent invention can be easily integrated into a digital circuit chip.Certain embodiments of the present invention can provide differentialvoltage signals for many applications, such as differential flashanalog-to-digital converter.

It is also understood that the examples and embodiments described hereinare for illustrative purposes only and that various modifications orchanges in light thereof will be suggested to persons skilled in the artand are to be included within the spirit and purview of this applicationand scope of the appended claims.

1. An apparatus for generating differential signals, the apparatuscomprising: a first operational amplifier receiving a first signal; asecond operational amplifier receiving a second signal; a firsttransistor, the first transistor including a first gate, a firstterminal, and a second terminal, the first gate being coupled to thefirst operational amplifier, the second terminal being configured tooutput a third signal; a second transistor, the second transistorincluding a second gate, a third terminal, and a fourth terminal, thesecond gate being coupled to the second operational amplifier, the thirdterminal being coupled to the first terminal through a first electricalelement, the fourth terminal being configured to output a fourth signal,a first difference between the fourth signal and the third signal beingsubstantially proportional to a second difference between the firstsignal and the second signal.
 2. The apparatus of claim 1 wherein: thefirst signal corresponds to one input signal; the second signalcorresponds to another input signal.
 3. The apparatus of claim 1wherein: the first signal corresponds to one input signal; the secondsignal is biased to a predetermined voltage level.
 4. The apparatus ofclaim 1 wherein the first electrical element comprises a first resistor.5. The apparatus of claim 4 further comprising a second resistor, thesecond resistor being coupled to the second terminal and the fourthterminal.
 6. The apparatus of claim 1, and further comprising acommon-mode feedback device, the common-mode feedback device beingconfigured to receive at least the third signal and the fourth signaland to adjust a first current supplier and a second current supplier. 7.The apparatus of claim 6 wherein a sum of the third signal and thefourth signal is substantially equal to two times of a common-modevoltage.
 8. The apparatus of claim 1 wherein the first transistor andthe second transistor are the same type.
 9. The apparatus of claim 1wherein the first transistor is an NMOS transistor, and the secondtransistor is another NMOS transistor.
 10. The apparatus of claim 1further comprising a first current supplier and a second currentsupplier.
 11. The apparatus of claim 1 wherein the first operationalamplifier and the second operational amplifier are the same type. 12.The apparatus of claim 1 wherein the first resistor and the secondresistor are characterized by substantially a same resistance value. 13.The apparatus of claim 1 wherein the first operational amplifierreceives a fifth signal from the first terminal, and the secondoperational amplifier receives a sixth signal from the second terminal.14. An apparatus for generating differential signals, the apparatuscomprising: a first operational amplifier receiving a first signal; asecond operational amplifier receiving a second signal; a firsttransistor, the first transistor including a first gate, a firstterminal, and a second terminal, the first gate being coupled to thefirst operational amplifier, the second terminal being configured tooutput a third signal; a second transistor, the second transistorincluding a second gate, a third terminal, and a fourth terminal, thesecond gate being coupled to the second operation amplifier, the fourthterminal being configured to output a fourth signal, the third terminalbeing coupled to the first terminal through a first electrical element,a first difference between the fourth signal and the third signal beingsubstantially proportional to a second difference between the firstsignal and the second signal; a first current supplier coupled to thesecond terminal; a second current supplier coupled to the fourthterminal; and a common-mode feedback device being configured to adjustthe first current supplier and the second current supplier.
 15. Theapparatus of claim 13 wherein: the first signal corresponds to one inputsignal; the second signal corresponds to another input signal.
 16. Theapparatus of claim 13 wherein: the first signal corresponds to one inputsignal; the second signal is biased to a predetermined voltage level.17. The apparatus of claim 13 wherein a sum of the third signal and thefourth signal is substantially equal to two times of the predeterminedvoltage level.
 18. The apparatus of claim 13 wherein the common-modefeedback device is biased to a predetermined voltage level.
 19. Theapparatus of claim 13 wherein a sum of the third signal and the fourthsignal is substantially equal to two times of a common-mode voltage.